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MARINE CRUSTACEANS. 695usually broad, forming a cover to the mouth, whereas those of the Thalassinidea and Anomalaare only exceptionally so. These characters, however, are in themselves hardly enough tonegative the supposition that the Crabs have originated from one of the other non-macrurousgroups. What does make it impossible to derive them fi"om the Anomala is the presencein the primitive crab Homolodromia of podobranchs on some of the legs, while an originfrom the Thalassinidea is equally untenable on account of the less primitive condition of thegrooves of the carapace and the reduction of the endopodite and flagellum of the firstmaxilliped in the latter group.We are now left with the Thalassinidea and the three divisions of the Anomala stillunaccounted for. Among these there can be no question that the Thalassinidea are, as awhole, the most primitive. Their straight symmetrical abdomen, with well-developed biramouslimbs on all the segments except the first, and the sixth pair broad and flat in all butThalassina, the presence in most of appendices internae, and in some of good pleura, thesimple legs of the fourth pair and sometimes also of the fifth, the rostrum usually welldeveloped,the moveable antennal scale seldom wholly lost, and the large gill-formulae ofmany genera, extending from the second maxilliped to the fourth leg, with mastigobranchs,podobranchs, arthrobranchs and sometimes pleurobranchs—all these point to the same conclusion.Nor can there be any doubt that the Paguridea and the Thalassinidea are closelyakin and have branched off from the same not very remote ancestor. The only differencesbetween the Axiidae and the primitive Pagurids with symmetrical abdomen such as Pylochelesare the presence of a pleurobranch on the last leg, a suture across the telson in some species(which is never found in Thalassinidea), better developed eyes', the loss of epipodites on thelegs, a reduced rostrum, subchelate legs of the fourth pair, and the branches of the last pairof abdominal limbs narrow and not adapted for swimming. The first three of these features,being primitive, show that the Paguridea are not to be derived from any of the existingThalassinidea and the remainder make it equally impossible to derive the latter group fromthe former. The two meet at a point below otir present horizon.The Galatheidea join the stem from which the last two groups arise before itsbifurcation, that is,- they are less closely akin to either of these than the latter are to oneanother. The fact that they may have epipodites on some of the legs, and the more primitiveshape of the rostrum and sixth pair of abdominal appendages make it impossible to placetheir ancestor within the present limits of the Paguridea. The suture on the telson andthe pleurobranch on the last leg remove it also from the Thalassinidea. And the generalshape of the body, depressed, with broad flat abdomen carrying long pleura and bent underthe thorax, is so different from the compressed body, with straight abdomen, of the primitivePaguridea and Thalassinidea that there can be little doubt that the ancestral Galatheid leftthe non-macrurous stock, after the Crabs indeed, but before it gave rise to the Thalassinidsor Hermit-crabs. As for the linea anomurica, this is found not only in the Galatheidea andPaguridea but also very distinctly in Callianassa novaebritanniae, and it is curious to noticethat it is wanting in the primitive Pagurids such as Pylocheles, and most Thalassinids as inAarius, and that in the Galatheid Aeglea, which is also primitive in many respects, the hinderpart is again wanting. Thus it would seem as though this line appeared only in the higherand more typical forms of each group. The Hippidea present a very difficult problem. On1 Such species as Axiopsis clypeatus, however, have the eyes well developed, though not so large as those of thePaguridea.89—2